1. Field of the Invention
The present invention relates to a semiconductor memory device, and particularly to a semiconductor memory device having a large storage capacity. In particular, the present invention relates to row-related control circuitry for controlling a row selecting operation in a refresh operation mode and a normal operation mode of a clock synchronous DRAM (Dynamic Random Access Memory) used, e.g., in a DRAM integrated on a common chip together with a logic or the like.
2. Description of the Background Art
FIG. 79 schematically shows a whole structure of a conventional clock synchronous semiconductor memory device. In FIG. 79, the clock synchronous semiconductor memory device includes a plurality of sub-memory arrays SMA0-SMA3, row decoders RD0-RD3 provided corresponding to sub-memory arrays SMA0-SMA3 for selecting rows of memory cells in corresponding sub-memory arrays, respectively, a column decoder CDA provided for sub-memory arrays SMA0 and SMA2 for producing a column select signal for selecting columns in these sub-memory arrays, a column decoder CDB provided corresponding to sub-memory arrays SMA1 and SMA3 for producing the column select signal for selecting columns in these sub-memory arrays, a data path DPA for transmitting data to and from memory cells in the column selected by column decoder CDA, and a data path DPB for transmitting data to and from memory cells in the column selected by column decoder CDB. Each of data paths DPA and DPB includes data input circuitry (an input buffer and a write buffer) and data output circuitry (an output buffer and a preamplifier).
Sub-memory arrays SMA0 and SMA1 form a bank BA#1, and sub-memory arrays SMA2 and SMA3 form a bank BA#0. Commonly to banks BA#1 and BA#0, there is arranged a main control circuit MCK that operates in synchronization with a clock signal CLK to receive an address signal ADD and a command CMD instructing an operation mode, and produces an operation control signal for banks BA#0 and BA#1. For bank BA#0, a sub-control circuit SCK0 is provided. For bank BA#1, a sub-control circuit SCK1 is provided. Main control circuit MCK produces an operation control signal for a designated bank in accordance with a bank address included in address signal ADD. Sub-control circuits SCK0 and SCK1 produce control signals for performing designated operations in accordance with the main operation control signal received from main control circuit MCK. These sub-control circuits SCK0 and SCK1 operate independently of each other in accordance with the operation control signal received from main control circuit MCK.
As shown in FIG. 79, the memory array is divided into two banks BA#0 and BA#1 so as to be activated and deactivated independently of each other in accordance with sub-control circuits SCK0 and SCK1, respectively. Therefore, the data access can be made to the banks in an interleaved manner, so that a penalty upon page switching is not caused, and fast access can be performed.
FIG. 80 schematically shows a structure of sub-memory arrays SMA0-SMA3 shown in FIG. 79. Sub-memory arrays SMA0-SMA3 have the same structure, and therefore, FIG. 80 shows only one sub-memory array as a representative.
In FIG. 80, sub-memory array SMA includes a plurality of memory blocks MB0-MB7, a sense amplifier bands SAB1-SAB7 arranged between memory blocks MB0-MB7, and sense amplifier bands SAB0 and SAB8 arranged outside memory blocks MB0 and MB7, respectively.
In memory block MB0, memory cells are arranged in rows and columns. In sense amplifier bands SAB0-SAB8, sense amplifier circuits are arranged corresponding to the columns of corresponding memory blocks MB0-MB7. Sense amplifier bands SAB0-SAB8 have a so-called “alternately arranged, shared sense amplifier structure”, in which the sense amplifier circuits are arranged alternately on the opposite sides of the columns of the corresponding memory blocks, and each sense amplifier circuit is shared between the adjacent memory blocks.
In the sub-memory array SMA, the row selecting operation is performed on a block basis. One of the memory blocks is designated by the block select signal produced in accordance with the block address includes in address signal ADD, and the row selection is performed in the selected memory block. Since sub-memory array SMA is divided into the plurality of memory blocks MB0-MB7, each of sub-control circuits SCK0 and SCK1 is divided into local control circuits corresponding to memory blocks MB0-MB7.
As shown in FIG. 80, a block dividing operation or a partial activation is performed in sub-memory array SMA and the memory blocks in an unselected state are maintained in a precharged state for reducing a current consumption.
For arranging the sub-memory array shown in FIG. 80, sense amplifier band SAB8 of bank BA#1 and sense amplifier band SAB0 of bank BA#0 are arranged adjacently to each other on a boundary between banks BA#0 and BA#1. Thus, the sense amplifier band of each bank can be activated and deactivated independently of those of the other bank.
FIG. 81 schematically shows a structure for a portion related to sub-control circuits SCK0 and SCK1 shown in FIG. 79. Sub-memory array SMA2 included in bank BA#0 includes memory blocks MB00-MB07. Sub-memory array SMA0 of bank BA#1 includes memory blocks MB10-MB17. The sense amplifier bands are arranged on the opposite sides of these memory blocks MB00-MB07 and MB10-MB17 in the column direction. In FIG. 81, these sense amplifier bands are depicted as rectangular regions, respectively.
Sub-control circuit CK0 includes local control circuits LCK00-LCK07 provided corresponding to memory blocks MB00-MB07, respectively, and sub-control circuit CK1 includes local control circuits LCK10-LCK17 provided corresponding to memory blocks MB10-MB17, respectively.
Main control circuit MCK produces a row-related control signal group BRC and a predecode block address signal PDA for the banks in accordance with externally applied command CMD and address signal ADD, and also produces internal clock signal CLK in accordance with an externally applied clock signal ECLK. Internal clock signal CLK generated from main control circuit MCK is applied commonly to local control circuits LCK00-LCK07 and LCK10-LCK17. Row-related control signal group BRC for the banks includes a row-related control signal BR0 for bank BA#0 and a row-related control signal BR1 for bank BA#1. Row-related control signal BR0 is applied commonly to local control circuits LCK00-LCK07, and row-related control signal BR1 is applied commonly to local control circuits LCK10-LCK17.
A predecode block address signal PBA is produced by predecoding a block address included in externally applied address signal ADD. Predecode block address signal PBA of 6 bits includes a predecode block address signal group PBG0 of 2 bits and a predecode block address signal group of 4 bits, and is applied commonly to banks BA#0 and BA#1. In FIG. 81, since each of banks BA#0 and BA#1 includes eight memory blocks, the predecode block address of 6 bits is produced. Predecode block address group PBG0 of 2 bits designates the memory blocks in an upper or lower half in each of banks BA#0 and BA#1. Predecode block address group PBG1 of 4 bits designates one memory block among these memory blocks in the upper half and the lower half in each of the banks. Therefore, each of local control circuits LCK00-LCK07 and LCK10-LCK17 receives one bit in each of these predecode block address bit groups PBG0 and PBG1.
Predecode block address signal PBA commonly designates the memory blocks in banks BA#0 and BA#1. In accordance with row-related control signal group BRC for the banks, the row-related control signals for the bank designated by the bank address included in address signal ADD is activated, and the operation related to row selection is performed in an activated bank.
For simplifying the figure, structures of sub-memory arrays SMA1 and SMA3 are not shown in FIG. 81. These sub-memory arrays SMA1 and SMA3 have structures similar to those of sub-memory arrays SMA0 and SMA2, and local control circuits LCK00-LCK07 and LCK10-LCK17 controls the row selecting operation therein.
Each of data paths DPA and DPB includes a write driver, a preamplifier and a data I/O buffer, and transmits data to and from the memory cell on a column selected by column decoder CDA.
As shown in FIG. 81, the row selection is performed on a block basis in each of banks BA#0 and BA#1, so that unselected memory blocks can be maintained in the precharged state, and the current consumption can be reduced.
The address signal (referred to as a “word line address signal”, hereinafter) for designating a word line must be applied commonly to all the memory blocks, or commonly to local control circuits LCK00-LCK07 and LCK10 and LCK17.
FIG. 82 shows an example of a structure of an input buffer in main control circuit MCK. Main control circuit MCK takes in externally applied command CMD and address signal ADD in synchronization with external clock signal ECLK (internal clock signal CLK). In FIG. 82, input buffer IB includes: an inverter IV that inverts clock signal (internal clock signal) CLK; a transmission gate XF1 that is turned on to pass input signal IN when clock signal CLK is at L-level; an inverter latch IL1 that latches the signal passing through transmission gate XF1; a transmission gate XF2 that is turned on to pass the signal latched by inverter latch IL1 when clock signal CLK is at H-level; and an inverter latch IL2 that latches the signal passing through transmission gate XF2 for producing an internal output signal OUT.
Transmission gates XF1 and XF2 are CMOS transmission gates, respectively, and are turned on/off in synchronization with clock signal CLK and a complementary clock signal generated from inverter IV. An operation of input buffer IB shown in FIG. 82 will now be described with reference to a signal waveform diagram shown in FIG. 83.
When clock signal CLK is at L-level, transmission gate XF1 is conductive, and inverter latch IN1 latches input signal IN. Meanwhile, transmission gate XF2 is non-conductive, and output signal OUT does not change.
When clock signal CLK rises to H-level, transmission gate XF1 is turned off, and input signal IN does not affect the latched signal of inverter latch IL1. When clock signal CLK rises to H-level, transmission gate XF2 is responsively turned on, and the signal latched by inverter latch IL1 is transmitted to inverter latch IL2, so that output signal OUT is produced. Accordingly, output signal OUT changes in synchronization with the rising of clock signal CLK.
Input buffer IB shown in FIG. 82 is provided in main control circuit MCK for each of address signal ADD and command CMD. Internal signals are produced in synchronization with rising of clock signal CLK, and therefore, the internal signals change in synchronization with the rising of clock signal CLK if a setup/hold time to clock signal CLK is ensured. Therefore, it is not necessary to consider a skew between these input signals, and it is possible to set the start timing of the internal operations faster.
FIG. 84 schematically shows line loads of the internal clock signal, row-related control signal and the predecode block address signal. In FIG. 84, internal clock signal CLK is transmitted by a clock driver DRV0 via a signal line SGL0. Row-related control signal BR (BR0 or BR1) is transmitted by a drive circuit DRV1 through a signal line SGL1. Predecode block address signal PB is transmitted by a drive circuit DRV2 via a signal line SGL2.
As shown in FIG. 81, the internal clock signal must be applied commonly to local control circuits LCK00-LCK07 and LCK10-LCK17, and therefore, signal line SGL0 have the largest load capacitance C0.
As for the row-related control signal BR, since all the local control circuits of the corresponding bank are coupled thereto, signal line SGL1 have a second largest load capacitance C1.
As for predecode block address signal PB, the local control circuits for the two memory blocks are connected in each bank to predecode block address signal bit group PBG1, so that the signal lines for the predecode block address signal bit group PBG1 has the smallest load capacitance C2. For predecode block address signal group PBG0, four local control circuits are connected in each bank to the signal lines for transmitting the predecode block address signal group PBG0. Therefore, a repeater may be arranged between the banks, whereby the load of the driver can be reduced, and the line load thereof can be made smaller than that for the row-related control signal. Since these signal lines SGL0-SGL2 have different line load capacitances C0-C2, their signal transmission delay times are different from each other, resulting in skews between these signals. In particular, these signals are transmitted unidirectionally along the column direction from main control circuit MCK toward local control circuit LCK17 at the remotest position. Therefore, a difference in signal transmission delay time also occurs between local control circuit LCK00 nearest to main control circuit MCK and local control circuit LCK17 remotest therefrom, and therefore a difference occurs in magnitude of the skew between the both.
FIG. 85 schematically shows a timing relationship among the input signals of local control circuits LCK00 and LCK17 as well as the externally applied signals, i.e., clock signal ECLK, address signal ADD and command CMD.
Main control circuit MCK is supplied with external clock signal ECLK, address signal ADD and command CMD. In synchronization with rising of external clock signal ECLK, main control circuit MCK takes in externally applied address signal ADD and command CMD, and produces predecode block address signal PBA and row-related control signal BR (BR0 or BR1). For local control circuit LCK00 nearest to main control circuit MCK, the smallest phase difference occurs between internal clock signal CLK and external clock signal ECLK. Main control circuit MCK produces row-related control signal BR0 and predecode block address signal PBA in synchronization with internal clock signal CLK, for transmission to local control circuit LCK00.
In local control circuit LCK00, signal line SGL0 transmitting internal clock signal CLK has large interconnection capacitance C0, and internal clock signal CLK arrives at local control circuit LCK00 with a slight delay to arrival of predecode block address signal PBA and row-related control signal BR0. In this case, however, the interconnection lines of these signals are short so that a skew between predecode block address signal PBA and internal clock signal CLK is small. If local control circuit LCK00 performs an operation synchronized with internal clock signal CLK at the above described timing, the setup time of the predecode block address signal PBA is insufficient so that a malfunction may occur.
In local control circuit LCK17 remotest from main control circuit MCK, internal clock signal CLK is transmitted with the largest delay due to the long interconnection length. Likewise, the delay times of row-related control signal BR1 and predecode block address signal PBA are larger than those for local control circuit LCK00, but are smaller than that of internal clock signal CLK to the local control circuit LCK7. In this case, a large phase difference occurs between predecode block address signal PBA and internal clock signal CLK, and thus a large skew occurs. In local control circuit LCK17, therefore, it is impossible to advance a timing for starting an internal operation, and the fast operation is impossible.
The operation start timing in each local control circuit may be determined depending on the distance from main control circuit MCK. However, such individual determination scheme complicates the circuit design. As external clock signal ECLK becomes faster, the timing adjustment time becomes an extremely short time, so that the timing adjustment must be performed extremely exactly. For the operation stability, the operation timing of the internal circuits may be determined in accordance with the worst skew conditions of local control circuit LCK17 remotest from main control circuit MCK. However, such a scheme based on the worst case makes the fast operation impossible.
In predecode block address signal PBA, predecode block address bit groups PBG0 and PBG1 are different in line load and delay time. Therefore, the timing at which all the predecode block address bits are made definite differs for different local control circuits, and an accurate decoding may not be performed.
FIG. 86 shows schematically a structure of main control circuit MCK. In FIG. 86, main control circuit MCK includes: a clock buffer 900 which receives externally applied clock signal ECLK, and produces internal clock signal CLK; a command input buffer 902 which takes in externally applied command CMD in synchronization with internal clock signal CLK from clock buffer 900; a row address input buffer 904 which takes in externally applied address signal ADD in synchronization with internal clock signal CLK; a row-related control signal generating circuit 905 which decodes the command received from command input buffer 902 in synchronization with internal clock signal CLK, and produces a row-related control signal BR0 for bank BA#0 in accordance with the result of decoding; a row-related control signal generating circuit 906 which decodes the command received from command input buffer 902 in synchronization with internal clock signal CLK, and produces a row-related control signal BR1 for bank BA#1 in accordance with the result of decoding; and a column related control circuit 908 which decodes the command received from command input buffer 902 in synchronization with internal clock signal CLK, and controls the operation of circuits related to data access (column selection).
Row-related control signal generating circuits 905 and 906 receive a bank address BAD from row address input buffer 904, and activates the row-related signal generating circuit provided for the bank designated by bank address BAD. Row-related control signal BR0 for bank BA#0 includes a row address decode enable signal RADE<0>, a word line drive timing signal RXT<0>, a bit line isolation instructing signal BLI<0>, a bit line equalize instructing signal BLEQ<0> and sense amplifier activating signals SON<0> and SOP<0>. Likewise, row-related control signal BR1 for bank BA#1 includes the corresponding signals RADE<1>, RXT<1>, BLI<1>, BLEQ<1>, SON<1> and SOP<1>.
According to the configuration shown in FIG. 86, column-related control circuit 908 controls data path DP performing input/output of data. However, column-related control circuit 908 also controls the operation of column decoders provided for banks BA#0 and BA#1. Data path DP includes a write driver, a preamplifier, a data input buffer and a data output buffer.
As shown in FIG. 86, main control circuit MCK includes row-related control signal generating circuits 905 and 906 corresponding to banks B#0 and BA#1, respectively. For providing more banks, therefore, the row-related control signal generating circuits must be increased in number, and therefore, a layout of the row-related control signal generating circuits in main control circuit MCK must be changed. Therefore, main control circuit MCK must be re-designed depending on a bank configuration. When the load on the signal line changes in re-designing, further re-design is required for adjusting an inter-signal skew. Therefore, it is difficult to accommodate the change in bank structure. If the banks increase in number, the row-related control signal generating circuits increase in number, and the signal lines for transmitting the row-related control signals increase in number, so that the interconnection region and the area occupied by the circuits increase, and the chip size increases.
In general, a storage capacity required for an embedded DRAM merged with a logic on a common chip depends on its application purpose, and it is required to change the number of memory blocks in accordance with the required storage capacity. In the embedded DRAM, memory cells store information in the form of electric charges accumulated in capacitors. Therefore, a refresh operation for rewriting or restoring the stored information of the memory cells must be performed periodically. If the memory blocks are variable in number as described above, a power is wasted in the refresh operation, and an access efficiency is lowered as well.
FIG. 87 schematically shows a structure of a portion related to refresh of one sub-memory array SMA. In FIG. 87, sub-memory array SMA includes, e.g., 12 memory blocks MB0-MB11. In each of memory blocks MB0-MB11, 512 word lines WL are arranged.
Local control circuits LCK0-LCK11 are arranged corresponding to memory blocks MB0-MB11, respectively. Local control circuits LCK0-LCK11 control operations related to row selection in corresponding memory blocks MB0-MB11, respectively. FIG. 87 representatively shows a block decoder BD, which produces a block select signal for selecting the corresponding memory block. Word line driver groups WDG0-WDG11 are provided corresponding to memory blocks MB0-MB11, respectively. Word line driver groups WDG0-WDG11 drive the word lines, which are selected in accordance with word line select signals applied from corresponding local control circuits LCK0-LCK11, to the selected state, respectively.
Each of word line driver groups WDG0-WDG11 includes word line drivers arranged corresponding to word lines in corresponding one among memory blocks MB0-MB11. The word line driver includes a main word line driver and a sub-word line driver if word line WL in the memory block is formed into a hierarchical word line structure. The sub-word line drivers are arranged corresponding to the sub-word lines, respectively, and are distributed in the corresponding memory block. The main word line driver is arranged corresponding to the main word line, and is located on one side of the corresponding memory block. For the sake of simplicity, FIG. 87 shows the word line driver group (main word line drivers) arranged on one side of the memory block.
For generating a block address for designating the memory block, there are provided: an address input buffer 910 which takes in an externally applied address signal ext<11:0> of 12 bits in synchronization with clock signal CLK; a refresh counter 912 performing a count operation in synchronization with a refresh activating signal REF that is rendered active for a predetermined period in response to the refresh instruction signal applied from a command decoder shown in FIG. 86, and generating a refresh address QA<11:0>; a multiplexer 914 which selects one of the internal address signal applied from address input buffer 910 and read address bits QA<11:0> applied from refresh counter 912 in accordance with refresh activating signal REF; and inverters IV2-IV0 which invert block address signal RA<11:9> of 3 bits received from multiplexer 914 to produce a complementary internal address signal. Multiplexer 914 applies an address signal RA<8:0> of 9 bits to a row decoder of a local control circuit (not shown).
Refresh counter 912 increments or decrements its refresh count value (refresh address) in synchronization with rising or falling of clock signal CLK when refresh activating signal REF is active.
In the structure shown in FIG. 87, the block address signal is formed of three bits RA<11:9>, and two memory blocks are simultaneously selected in twelve memory blocks MB0-MB11. In the arrangement shown in FIG. 87, a block address “000” is allocated to memory blocks MB0 and MB6, a block address “001” is allocated to memory blocks MB1 and MB7, and a block address “010” is allocated to memory blocks MB2 and MB8.
Also, a block address “011” is allocated to memory blocks MB3 and MB9, and a block address “100” is allocated to memory blocks MB4 and MB10. A block address “101” is allocated to memory blocks MB5 and MB11.
When the refresh is executed in this structure, therefore, word lines WL are simultaneously driven to the selected state in the two memory blocks. If refresh counter 912 are configured to accommodate 16 memory blocks, count value QA<11:9> of this block address must return to the initial block address “000” after changing from “000” to “101”. For applying refresh counter 912 to the structure including 16 memory blocks, a block address of four bits must be generated. Depending on a required specification, the refresh is executed on a memory block basis (i.e., a memory block at a time) or is executed in a unit of two memory blocks (i.e., two memory blocks at a time). In order to adapt to various specifications, a block address signal of four bits is required for executing the refresh on the memory block basis.
Therefore, if refresh counter 912 of the above structure is used, the count value of refresh counter 912 must be counted up from “110—00000 0001” to “1111—1111 1111” when the refreshing for memory blocks MB5 and MB1 is completed. Thus, a refresh command must be successively input until the count-up of refresh counter 912. Thus, in spite that refreshing of the memory cells in the whole space of sub-memory array SMA is completed by counting up refresh address QA<11:0> from “000—000000000” to “101—111111111”, the refresh command must be further applied (512×2) times for setting the refresh address to the initial value.
Accordingly, in the structure formed of 12 memory blocks, the refresh command must be applied (512×8=4 K) times in total although the refresh of memory cells in the whole space is completed by applying the refresh command (512×6=3 K) times. Therefore, it is necessary to input the refresh command, which does not cause the refresh but is used merely for operating the refresh counter. During this period of input of refresh command, external access is impossible, and in addition, refresh counter 912 wastes the power.
It may be considered to provide refresh counter 912 with a reset circuit for resetting the count value to an initial value of “000 000 000” when the count value reaches “101—111111111”. However, such reset circuit can be employable only for a structure formed of 12 memory blocks, and cannot be employed for a structure of 16 memory blocks.
If the number of memory blocks is equal to an integer of a power of 2, input of an unnecessary refresh command can be prevented by resetting the count value of refresh counter 912 at the time of change in any one of block address bits RA<11:9>. However, if the number of memory blocks is equal to, e.g., 12, and is not equal to an integer of a power of 2, as shown in FIG. 87, it is required to program the address of the final refresh memory block in the refresh sequence using a fuse program circuit or the like, for resetting refresh counter 912 in accordance with matching/mismatching between the block address bits output from refresh counter 912 and the programmed refresh block address. A layout area of a fuse element is larger than that of a transistor element. Also, the fuse program circuit requires a large layout area for preventing short-circuit due to a fragment produced upon blowing off the fuse. Therefore, if the fuse program circuit is used, a circuit scale of the refresh counter increases, and an area occupied by the refresh counter increases.
If the number of memory blocks increases to 16 in the structure shown in FIG. 87, the number of bank address signal bits increases to 4 bits, so that it is necessary to change the combination of the block address signal bits for selecting a block decoder BD. Accordingly, for changing the input of the address signal of block decoder BD in each local control circuit, an interconnection pattern must be changed in accordance with the number of memory blocks, resulting in a problem that the memory blocks cannot be expanded easily. In other words, it becomes difficult to change the number of memory blocks by optimising the layout of word line driver group WDG and local control circuit LCK for one memory block, and repeating this optimised layout. The interconnection layout of the block address signal bits must be changed in accordance with change in number of the memory blocks. As a result, it is impossible to adapt to the change in number of memory blocks and thus change in storage capacity without difficulty.
In the case of changing the refresh cycle in accordance with a required specification, i.e., changing the number of word line(s) to be selected simultaneously in the refresh operation, the greatest block address of the refresh address counter also changes, and the block address in the refresh operation for the memory blocks must be changed. Therefore, it is impossible to accommodate for the change in refresh cycle, similarly to the above case.
For ensuring reliability of the semiconductor memory device, the semiconductor memory device must be tested after production. For this test, the test control signal must be applied in synchronization with a clock signal if the semiconductor memory device is of a clock synchronous type. Timing margins of each external signal and data for the clock signal as well as access times can be tested in the semiconductor memory device of the clock synchronous type described above.
However, it is impossible to test a timing margin for to an internal control signal when internally selecting a memory cell. For example, it is impossible to measure a so-called RAS precharge time tRP through measurement of a time period required for internal precharge. In this case, the minimum measurable unit time of the RAS precharge time is equal to one clock cycle time of the clock signal even if the precharge command instructing the precharge and the row active command instructing the memory cell selection are successively applied. This is because the command is applied in synchronization with the clock signal. With a fast operable tester, the clock cycle period can be reduced, and the internal timing can be measured. However, such a fast test apparatus is expensive, and increases a test cost per device so that the semiconductor memory device becomes expensive.